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In dutch we have clear rules how to disassemble a sentence into its seperate parts.
In no korean textbook, or on any website could I find a decent rule to explain how to find what each word/word structure means.

Let me give you an example of what I mean.

Example Sentence:
"He is doing his homework"

subject: Who or what is doing his homework = He
so " He" is the subject.

To find the accusative the question becomes
Who or What is he doing: = his homework
So "his homework" is the accusation.

if not mistaken this would mean in korean particle grammar.

He는 is doing his homework를.

does anyone know or can come up with a clear set of defenitions when u need to use 은/는, 이/가, 을/를.
expecially the 2 subject particles are a problem.

I'm afraid I don't fully understand your question. Or it seems to me that you're answering your own question, because you already know a subject needs 은/는 or 이/가 and an object needs 을/를.
Korean is a SOV language but in practice a sentence can be OSV or even SVO in spoken language, partly because particles supplement the changed word orders.
It would be easier to answer if you could come up with more example sentences you find problematic.